A Mechanism for Stimulating AGN Feedback by Lifting Gas in Massive Galaxies

TL;DR

This paper proposes a model where rising X-ray bubbles lift low entropy gas in massive galaxies, leading to thermal instability and molecular cloud formation, which fuels feedback loops and stabilizes the galaxy atmosphere.

Contribution

It introduces a new mechanism for AGN feedback involving uplifted low entropy gas and thermal instability, contrasting with previous precipitation models.

Findings

Molecular clouds are observed in wakes of rising X-ray bubbles with sub-free-fall velocities.

The model explains the observed cooling time threshold of ~5x10^8 years for thermal instability.

Contrasts with precipitation models, showing better consistency with observations.

Abstract

Observation shows that nebular emission, molecular gas, and young stars in giant galaxies are associated with rising X-ray bubbles inflated by radio jets launched from nuclear black holes. We propose a model where molecular clouds condense from low entropy gas caught in the updraft of rising X-ray bubbles. The low entropy gas becomes thermally unstable when it is lifted to an altitude where its cooling time is shorter than the time required to fall to its equilibrium location in the galaxy i.e., t_c/t_I < 1. The infall speed of a cloud is bounded by the lesser of its free-fall and terminal speeds, so that the infall time here can exceed the the free-fall time by a significant factor. This mechanism is motivated by ALMA observations revealing molecular clouds lying in the wakes of rising X-ray bubbles with velocities well below their free-fall speeds. Our mechanism would provide cold gas needed to fuel a feedback loop while stabilizing the atmosphere on larger scales. The observed cooling time threshold of ~ 5x 10^8 yr --- the clear-cut signature of thermal instability and the onset of nebular emission and star formation--- may result from the limited ability of radio bubbles to lift low entropy gas to altitudes where thermal instabilities can ensue. Outflowing molecular clouds are unlikely to escape, but instead return to the central galaxy in a circulating flow. We contrast our mechanism to precipitation models where the minimum value of t_c/t_ff < 10 triggers thermal instability, which we find to be inconsistent with observation.